One of the most important chemical structure in the study of elements is Phosphine (PH3). This is a colourless gaseous substance discovered by ‘Philippe Gengembre’ in 1783 and is often confused with the study of Phosphene. Let us hence define what is Phosphine with a clear-cut idea about its structure and function with formulas for the same. We will also learn about the laboratory preparation of Phosphine and how noxious this substance is brief. 

What Is Phosphine?

Present in the chemical group called ‘organophosphorus’, Phosphine (IUPAC name: Phosphane) is an element that is a highly toxic gas with flammable properties. A French chemist Philippe once generated a regular supply of heat over phosphorous  (P4) with an aqueous solution of potassium carbonate; the result was a colourless gaseous substance. This highly unpleasant smell of gas was coined as ‘Phosphine’ due to its rotting fish odour. Phosphine is classified in the periodic table as in the pnictogen hydride section. Phosphine (PH3) is essential to the biochemical cycle, even though it possesses critical chemical properties with an unstable compound concentration in the atmosphere. 

Structure of Phosphine

The entire molecular structure of Phosphine is a trigonal pyramid. The diagram of Phosphine is studies based on the Lewis structure. Lewis structure is also called electron dot structures and is a picture to represent the lone pair of electrons and bonds with atoms or molecules. This can be grasped quickly using the following pointers:

  • Phosphine lewis structure has 8 valence electrons. 

  • Since both N (Nitrogen) and P (Phosphorous) are present in the same group in the periodic table, the structure of PH3 is similar to NH3 (Ammonia).

  • However, the electro-negativity of Phosphorus is lesser than that of Nitrogen.

  • 93.5 is the bond angle between H-P-H regions in the structure of Phosphine.

  • The bond length in P-H is 1.42 A.

Phosphine is considered to be a Lewis base. This is due to the presence of a non-bonding electrons pair on the P shell which may be donated.

Preparation of Phosphine

The preparation of Phosphine gas begins by mixing dilute Hydrochloric Acid (HCl) or water with calcium phosphide (Ca3P2). The below formula represents this preparation.  

Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3

Ca3P2 + 6HCl → 3CaCl2 + 2PH3

As you might have noticed, the other results along with Phosphine are Calcium Hydroxide (in the case of water) and Calcium Chloride (in the case of hydrochloric acid).

The laboratory preparation of Phosphine goes by the following case:

P4+3NaOH+3H2O → PH3 + 3NaH2PO2

Here, White Phosphorus has to be heated with concentrated Sodium Hydroxide to gove out Phosphine. Note that this preparation has to be undertaken in an inert atmosphere to make use of Carbon dioxide (CO2).


Physical and chemical properties of Phosphine

Physical properties of Phosphine are given below:

  • Formula - PH3

  • Critical Temp. (°F) - 124.9

  • Density of the gas @ 70°F 1 atm (lb/ft3) - 0.0885

  • Specific Gravity - 1.203

  • Melting Point (°F) - 208.8

  • Boiling Point (°F) - 126.0

  • Molecular Weight (lb/mol) - 34.00

  • Critical Pressure (Pc) - 947.9

  • Liquid Density @ 70°F (lb/ft3) - 35.50

Understand the chemical properties and functions of Phosphine gas with the following pointers in mind:

  • Exploding nature is high if disclosed to oxidising agents. 

  • High dissolution property in certain solvents and sparingly dissolvable in water.

  • An extremely noxious gas.

  • According to chemical safety, Phosphine possesses the characteristic to cause high environmental damage. 

  • Acts as a lewis base when its lone electron is donated in reaction with hydrogen iodide. 

  • Accurately toxic and highly flammable. 

  • Smells more like a spoiled fish or a clove of garlic.

  • When in contact with even minimal amount of Phosphine, it can trigger problems such as dizziness, jaundice, loose bowels, kidney and liver damager, inflammation of nasal cavity, fatigue, regurgitating, cerebral pain, convulsions, coma, shock and more.

  • Other names to denote PH3 - Phosphorus hydride, Fosforowodor, Phosphorwasserstoff, Trihydrogen phosphide, Gas-ex-B and Hydrogen phosphide.

Uses of Phosphine

Accounting to the uses of Phosphine, there are a few important roles that PH3 plays in different areas of work. The nature of combustion is spontaneous in PH3 and is hence useful as in the Holme’s signal for the same. Plastic industries make use of PH3 to create new flammable kits. Phosphine is a Dopant (substance useful to form ceratin electrical component) in semiconductor factories. This substance is also preferred in the preemption of incendiaries and flame retardants. In the process of fumigating grains and animal feed, Phosphene plays the key role.

FAQ (Frequently Asked Questions)

1. How is phosphine prepared?

Phosphine is prepared either by mixing HCl or water with calcium phosphide. It can also be prepared in a clinic strictly under inert conditions (for CO2).

2. Is phosphine a non-polar element?

No. Phosphine is a polar element but with non-polar molecule bonds. This is understood from its chemical formula PH3 (3 Hydrogen bonds with a lone pair of electrons).

3. Why is the smell of Phosphine compared to that of a rotten fish?

The smell of phosphine is more like a rotten or garlic, with regards to its substituted phosphine and diphosphine.

4. Can Phosphine be considered as a greenhouse gas?

Phosphine might possess the properties of greenhouse gas since PH3 reacts chemically with other hydroxyl radicals (·OH) similar to many reducing agents.

5. List any 2 safety precautions for careful usage of Phosphine?

To use Phosphine with safety, one should wear explosion-proof equipment along with safety gloves and shoes as well. Using respirators can also come handy during non-ventilated conditions.